CMOS pixel using vertical structure and sub-micron CMOS process
View Patent ↗A CMOS pixel responsive to different colors of optical radiation without the use of color filters is described. A deep N well is formed in a P type silicon substrate. An N well is then formed at the outer periphery of the deep N well to form a P well within an N well structure. Two N + regions are formed in the P well and at least one P + region is formed in the N well. A layer of gate oxide and a polysilicon electrode is then formed over one of the N + regions. The PN junction between the deep N well and the P type silicon substrate is responsive to red light. The PN junction between the deep N well and the P well is responsive to red light. The PN junction between the P well and the N + region which is not covered by polysilicon and the PN junction formed by the N well and the P + region are responsive to green or blue light. The PN junction formed by the junction between the P well and the N + region which is covered by polysilicon is responsive to green light. The green signal is subtracted from the blue/green signal to produce a blue signal.
1. A pixel structure, comprising:
a substrate of a first conductivity type silicon;
a first well of a second conductivity type silicon formed in said substrate, wherein said first well is a distance below the top surface of said substrate, and the polarity of said second conductivity type is opposite to the polarity of said first conductivity type;
a second well of said second conductivity type silicon formed in said substrate, wherein the top surface of said second well is the top surface of said substrate and said second well has an inner periphery and an outer periphery;
an overlap region of said second conductivity type silicon between said first well and said second well, wherein the size of said overlap region is such that said overlap region can be depleted to electrically isolate said first well from said second well or not depleted to electrically connect said first well and said second well as desired;
a third well of said first conductivity type silicon in the region within said inner periphery of said second well and between the top surface of said substrate and said first well, wherein said first well and said second well isolate said third well from said substrate;
a first region of said second conductivity type silicon formed in said third well, wherein the top surface of said substrate forms the top surface of said first region;
a second region of said second conductivity type silicon formed in said third well, wherein the top surface of said substrate forms the top surface of said second region; and
a third region of said first conductivity type silicon formed in said second well, wherein the top surface of said substrate forms the top surface of said third region.
2. The pixel structure of claim 1 wherein said first conductivity type silicon is P type silicon.
3. The pixel structure of claim 1 wherein said second conductivity type is N type silicon.
4. The pixel structure of claim 1 wherein the junction between said first well and said substrate has a good response to radiation having wavelengths at or near the wavelength for red light.
5. The pixel structure of claim 1 wherein the junction between said first region and said third well and the junction between said third region and said second well have a good response to radiation having wavelengths at or near the wavelength for blue light.
6. The pixel structure of claim 1 wherein the junction between said first region and said third well and the junction between said third region and said second well are used to reset the pixel.
7. The pixel structure of claim 1 , further comprising:
a layer of dielectric formed over said second region of said second conductivity type silicon; and
a layer of polysilicon formed over said layer of dielectric and covering said second region of said second conductivity type silicon.
8. The pixel structure of claim 7 , wherein the junction between said second region and said third well has a good response to radiation having wavelengths at or near the wavelength for green light.
9. The pixel structure of claim 1 wherein the amount of carrier depletion in, and thereby the conductivity of, said overlap region is determined by the potential of said third well.
10. A CMOS pixel structure, comprising:
a substrate formed of P type epitaxial silicon;
a first N well, formed of N type silicon, in said substrate, wherein said first N well is a distance below the top surface of said substrate;
a second N well formed of N type silicon in said substrate, wherein the top surface of said second N well is the top surface of said substrate and said second N well has an inner periphery and an outer periphery;
an overlap region of N type silicon between said first N well and said second N well, wherein the size of said overlap, region is such that said overlap region can be depleted to electrically isolate said first N well from said second N well or not depleted to electrically connect said first N well to said second N well as desired;
a P well formed of P type silicon within the inner periphery of said second N well and between the top surface of said substrate and said first N well;
a first N + region formed of N + type silicon in said P well, wherein the top surface of said substrate forms the top surface of said first N + region;
a second N + region formed of N + type silicon formed in said P well, wherein the top surface of said substrate forms the top surface of said second N + region; and
a first P + region, formed of P + type silicon in said second N well, wherein the top surface of said substrate forms the top surface of said first P + region.
11. The CMOS pixel structure of claim 10 wherein the PN junction between said first N well and said P type substrate responds to red light.
12. The CMOS pixel structure of claim 10 wherein the PN junction between said first N + region and said P well and the PN junction between said first P + region and said second N well respond to blue or green light.
13. The CMOS pixel structure of claim 10 wherein the PN junction between said first N + region and said P well and the PN junction between said first P + region and said second N well are used to reset the pixel.
14. The CMOS pixel structure of claim 10 , further comprising:
a layer of dielectric formed over said second N + region; and
a layer of polysilicon formed over said layer of dielectric and covering said second N + region.
15. The CMOS pixel structure of claim 14 , wherein the PN junction between said second N + region and said P well responds to green light.
16. The CMOS pixel structure of claim 10 wherein the amount of carrier depletion in, and thereby the conductivity of, said overlap region is determined by the potential between said P well and said substrate.
17. The CMOS pixel structure of claim 10 further comprising a PMOS transistor formed in said second N well.
18. The CMOS pixel structure of claim 10 further comprising an NMOS transistor formed in said P well.
19. The CMOS pixel structure of claim 10 wherein during the reset cycle the potential between said P well and said substrate is adjusted so that said overlap region is not depleted and the potential of said first N well is set to a reset voltage.
20. The CMOS pixel structure of claim 10 wherein during the charge accumulation cycle the potential between said P well and said substrate is adjusted so that said overlap region is depleted and said first N well is isolated.
21. The CMOS pixel structure of claim 10 wherein during the readout cycle the potential between said P well and said substrate is adjusted so that said overlap region is not depleted and the potential of said first N well is transferred to said second N well.
22. A pixel circuit, comprising:
a P type substrate connected to ground potential;
an N well formed in said P type substrate, wherein the PN junction between said N well and said P type substrate forms a first diode;
a P well formed in said N well wherein a part of said N well lies below said P well and the PN junction between said P well and said N well forms a second diode;
a PMOS transistor formed in said N well, wherein the drain of said PMOS transistor is connected to ground potential;
a first NMOS transistor formed in said P well wherein the drain of said first NMOS transistor is connected to a high potential, V DD ;
a second NMOS transistor wherein the drain of said second NMOS transistor is connected to the source of said first NMOS transistor;
a third NMOS transistor wherein the source of said third NMOS transistor is connected to the source of said PMOS transistor;
a third diode connected between said N well and a first reset voltage node, wherein the anode of said third diode is connected to said N well and the cathode of said third diode is connected to said first reset voltage source; and
a fourth diode connected between said P well and a second reset voltage node, wherein the cathode of said third diode is connected to said P well and the anode of said third diode is connected to said second reset voltage source.
23. The pixel circuit of claim 22 wherein said first NMOS transistor and said PMOS transistor are operated as source follower transistors.
24. The pixel circuit of claim 22 wherein said second NMOS transistor and said third NMOS transistor are formed in said P type substrate.
25. The pixel circuit of claim 22 wherein said third diode is formed in said N well.
26. The pixel circuit of claim 22 wherein said fourth diode is formed in said P well.
27. The pixel circuit of claim 22 wherein during the reset cycle said first reset voltage node is set at a high potential, V DD , and said second reset voltage node is set at ground potential.
28. The pixel of claim 22 wherein during the charge integration cycle said first reset voltage node is set at ground potential and said second reset voltage node is set at a high potential, V DD .
29. The pixel of claim 22 wherein during the readout cycle a red/green signal is taken form the drain of the third NMOS transistor.
30. The pixel of claim 22 wherein during the readout cycle a blue/green signal is taken from the source of the second NMOS transistor.